Basically, the king of these giant people thinks that the history and governmental system of the British proves that they are horrible people. After Gulliver explains British history and government, the king tells him

I cannot but conclude the Bulk of your Natives, to be the most pernicious Race of little odious Vermin that Nature ever suffered to crawl upon the Surface of the Earth.

In general, what the king says is that British history is just too full of violence and other evils. He says it shows that the British are, among other bad things, hypocritical, lustful, and malicious.

Swift tells us of a number of questions that the king asks Gulliver. These can be found in Chapter 6 and they can show you specific things that the king thinks are bad.